A large number of heart valve prostheses have been designed for implantation into the human heart valve to replace defective natural heart valves. Although heart valves are merely check valves which open to allow blood flow in a normal or downstream direction and close to prevent regurgitation of blood flowing in a reverse or upstream direction, a good deal of effort has gone into designing and improving the details of heart valves which are designed for implantation in the human body.
In its open position, a valve should provide a passageway which is large and which has good flow characteristics so that blood flows freely therethrough with a minimum of drag and eddy currents. A heart valve should be responsive to blood flow to quickly open in response to blood flow in the normal direction and to close quickly in response to backflow to prevent regurgitation of blood. The heart valve must, of course, be biocompatible and thromboresistant, and in this regard, it is important that all surfaces be well washed by blood to prevent stagnation which might lead to eventual clotting. The opening and closing of the valve should be sufficiently soft so as not to cause hemolysis (breaking of blood cells).
Heart valves, which must withstand countless openings and closings, should be designed to avoid excessive wear, and particular care should be exercised so that the load-bearing surfaces, such as the pivot points and stops, do not wear excessively during the life of the patient. Experience with mechanical heart valves has shown that the major portion of heart valve wear occurs at the moment of closing. The forces during closure are great since both a pressure gradient and substantial fluid momentum (water hammer) act on the valve members at this instant.